Event Sourcing, Domain Driven Design, and Command Query Responsibility Segregation – we hear all of these technologies used together frequently, but how do they actually work together? How do you manage complex co-ordination in a CQRS system? In this talk, we will discuss a real world example of DDD with ES and CQRS written in F# - a functional first language on the .NET Framework. We’ll take a deep dive into the F# algebraic type system that constructs the domain model. Also, the explanation of the abstract notion of a DDD aggregate root compared to the CQRS implementation of an aggregate root. Finishing with how sagas and triggers facilitate poly-aggregate communication.

1. RealWorld Event Sourcing
and CQRS
Matthew Hawkins | @mattchawkins

2. Agenda
• Overview of my journey into CQRS, ES, and F#
• Explore different patterns, practices and architectures
• 4 Segments
1) Event Sourcing
2) F#
3) Domain Driven Design
4) CQRS
• Disclaimer:This is what I’ve found works (or doesn’t) for us.
• Fast paced, will be covering a lot of information

3. Traditional Monolithic System
• Concurrency issues (Locking)
• CRUD Design
• Multiple Models
• Lacking developer discipline
• Typically not very scalable
Client
Model-View-Controller
RDBMS

4. CQRS/DDD/ES System
Client
ReSTful API
Command Model
Consumes
Writes
Query Model
Event
Store
Denormalized
Projection
Publishes
Reads
• Reads/Writes Segregated
• Concurrency Handling
• Clear separation of concerns (Read
vs Domain Model)
• Requires developer discipline
• High-throughput, scalable

5. Part One
“A way of representing state by storing the
history of the application’s state”
What is Event Sourcing?

6. What is an Event Store?
An immutable append-only transaction log.
Stock
Traded
Account
Updated
Stock
Traded
Account
Updated
Account
Updated
Time

7. RDBMS vs ES (Event Sourcing)
Relational Database Event Sourcing
State Current State
Tables Streams
Schemas Buckets

8. … what is Event Sourcing? (Cont.)
• Event Sourcing is a practice
• Persistence occurs in an “event store”
• Related events compose an “event stream”
• Streams are in chronological order
• Capability to project / rehydrate models
• Majority of RDBMS use a transaction log behind-the-scenes

9. What is an event?
• Events are state delta’s
• Events are immutable
• Transient or persisted
• Wrapped in a message (Header, Payload)
• Possibly consumed by 3rd parties
• Can be classified as a Domain, Infrastructural or System event
• Most events are past-tense behaviors of the domain (Ex: StockTraded)

10. What does an event look like?
• Domain Event Example (In F#):

11. EventVersioning
• Events are persisted in a serialized representation
V1
V2

12. Event Stream
• Event Stream : Grouping of related events
Stock
Traded
Account
Updated
Stock
Traded
Account
Updated
Account
Updated
Time
100101101 100100 Stream ID

13. Functionalities of aTransaction Log
• Slice stream
Contiguous subset of the stream
• Probe stream
Returns max score/offset of stream used for concurrency/state version management
• Append stream
Writes to the end of a stream
• Scan stream
Returns a specific subset of the stream (example: scan all “account updated”)
• Get stream
Returns all events from a specific stream, typically used for model rehydration

14. Rehydration
• The process of querying an event stream (GET) and “applying” it to the
model.
• Allows deterministic rehydration to a given point in time, commonly;
“Current State”

15. Projections
• Denormalized representation of the model
• Eventually consistent to the write model
• Projections are typically aligned to a specific screen on the UI
• May be organized by Aggregate
Read Model Event Read Model

16. F#
• F# is a mature, open source, cross-platform, functional-first programming
language.
• Why F#? (With DDD & ES)?
• Self documenting, expressive code
• The type system (algebraic, composable) prevents incorrect state
• Immutable out-of-the-box (Structural equality for value types)
PartTwo

17. F#Type System Examples
• Comparison
Unit of Measure
RecordType
UnionType
Record withTuples

18. F# Competitive Advantage
• The code serves as a self-documenting ubiquitous language
• WAY LESS CODE!! Once you learn it
• You can design your domain model to not allow incorrect states at compile
time
• Forced to handle errors (at compile time)
• No nulls! (Alternatives such as Options<‘T> and Choice<‘T1,’T2..>)

19. Domain Driven Design
• What is DDD?
• When and why DDD?
• What it provides you and your team?
PartThree

20. Bounded Contexts
• One large model vs. Bounded Contexts
• Can be aligned to an autonomous service
• Can’t start a DDD project without a BC (Bounded Context)
Trading Hedging
Accounting Management

21. Entities
• Have identity
• Equality is based off identity
• Have behavior associated with them

22. Values
• No identity
• Immutable
• Structurally equatable
• Explicit type (primitives -> domain concept)
• May have localized behavior

23. Aggregates
• Abstract notion of
domain types
• Collection of entity
and value types
• Not-concrete
• Load/save aggregates

24. Aggregate Roots (DDD Context)
• All interaction with the aggregate (entities and values) goes through the
aggregate
• An AR is an Entity
• Manages consistency within the aggregate (all state change)
• Not to be confused with CQRS Aggregate root

25. Sagas
• Responsible for long-running business processes , “workflows”
• Allows for eventually consistent communication between aggregates
• Consume domain events, fire commands
• Events may be transient
TradeOrderCreated TradeExecuted TransactionRecieved

26. Command-Query Responsibility
Segregation (CQRS)
• What is CQRS?
• When and why CQRS?
Part Four

27. System Overview
Client
ReSTful API
Command Model
Consumes
Writes
Query Model
Event
Store
Denormalized
Projection
Publishes
Reads

28. ReSTful API
HTTPVerb API Action
GET Queries the Read Model
POST Command on Queue -> (Create Command)
PUT Command on Queue -> (Update Command)
DELETE Command on Queue -> (Delete Command)
API Request
API Command
Command Handler

29. Commands
• Instantiated in the API
• No validation is performed
except for deserialization
• Placed on the queue to go
to the Handler

30. Triggers
• API Commands are mapped to triggers in the Command Handler
• Triggers are granular domain actions.When fired against the Aggregate
root, yield an event. (Fire : ‘state-> ‘trigger -> ‘event)
• Allow for explicit pathways of state change (Business rules, clear model)

31. Command Handlers

32. Aggregate Fire Function
‘state -> ‘trigger -> ‘event

33. Events
• Domain events represent the delta state change of the model
• The “Fire” function on an Aggregate root yields an event
• Events are serialized and stored in the event store
• Once persisted, they’re broadcasted to interested parties

34. Event Handlers
• Consume events dispatched from the Event Store
• Commonly used for:
• Updating the Read Model
• Notifying a 3rd party system
• Sagas

35. CQRS Aggregate Roots
• Gatekeeper for:
• Triggers interacting the with Aggregate
• Model Rehydration (Events)

36. CQRS Aggregate Root

37. Sagas,Triggers, and Poly-Aggregate
Communication• Sagas
• Provide an eventually consistent way of communicating between aggregates
• Best used when the process :
• Is “long running” (greater than a few seconds)
• Has a lot of business rules
• Interacts with 3rd party systems
• Trigger Pattern
• Provide a consistent way of multiple aggregates communicating at the same time
• Best used when:
• Two or more aggregates need to be validated/interact atomically
• Triggers are still used with one aggregate at a time!

38. Projections
• Denormalized representations of the domain model
• Typically correlate to 1) One UI screen 2) An aggregate
• Very similar to domain model rehydration (Apply: ‘state -> ‘event -> ‘state)
• Eventually consistent. Should not be used on the “Command/write side”
• Can be different than the write model

39. Read Model Query Cache Projection
Event Handlers
Push to ->
API GET->

40. • geteventstore.com
• NEventStore (++ CommonDomain)
• Domain Driven Design by Eric Evans
• Fsharp.org
• GalaxE Solutions (@GalaxESolutions)
• QuickenLoans (@QLTechnology)
• Matt Hawkins (@MattCHawkins)
Still Interested?